The disclosure of Japanese Patent Application No. 2000-151896 filed on May 23, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a continuously variable transmission that performs speed shift by using a hydropneumatic actuator and, more particularly, to a shift control apparatus and a shift control method for controlling the speed ratio.
2. Description of the Background
A continuously variable transmission is known in which a transfer member is provided between a rotatable input member and a rotatable output member in such a fashion that the transfer member contacts the input and output members, and in which a speed shift operation is performed by changing the positions of contact of the transfer member with the input member and the output member.
An example of such a continuously variable transmission is disclosed in Japanese Patent Application Laid-Open No. HEI 3-103654. In the apparatus described in the patent application, each of the input and output pulleys is made up of a stationary sheave and a movable sheave that have a generally cone or frustum shape. The stationary and movable sheaves are disposed so that peripheral surfaces thereof having a conical shape or the like face each other. A belt is sandwiched between the peripheral surfaces of the sheaves. By changing the interval between the sheaves of each pulley, the position at which the belt passes on the pulley, that is, the belt engagement radius on the pulley, can be varied. By changing the sheave interval of the input-side pulley, the belt passing position is controlled so as to control the ratio between the rotation speed of the input pulley and the rotation speed of the output pulley, that is, the speed ratio. The sheave interval is changed by moving the movable sheave through the use of a hydropneumatic actuator.
The operating fluid used for the hydropneumatic actuator in most cases is a liquid and, more particularly, a polymeric hydrocarbon generally termed xe2x80x9coil.xe2x80x9d This operating fluid greatly changes in viscosity depending on temperature. When the temperature decreases, the viscosity of the operating fluid increases. If the viscosity increases, the flow passage resistance of the operating fluid increases, so that the amount of the operating fluid supplied to or discharged from the actuator decreases. This reduces the stroke speed of the actuator, giving rise to the problem that a predetermined value of the changing rate of the speed ratio (hereinafter, referred to as xe2x80x9cspeed ratio-changing ratexe2x80x9d) cannot be achieved, therefore degrading the speed shift responsiveness.
The invention has as an object to solve the aforementioned problem.
It is another object of the invention to perform an appropriate shift control when an operating fluid of a hydropneumatic actuator used in a continuously variable transmission has a low temperature.
In order to address the aforementioned and other objects, a continuously variable transmission shift control apparatus in accordance with a first mode of the invention sets a target input rotation speed within a second setting range that is narrower than a first setting range of the target input rotation speed provided for use for a normal temperature of the operating fluid, if the temperature of the operating fluid is lower than the normal temperature. That is, when the operating fluid temperature drops to such an extent as to affect the speed ratio-changing rate, the range of change of the target input rotation speed is limited to a range that is narrower than the range of change set for the normal temperature.
Therefore, even if the speed shift responsiveness deteriorates due to low operating fluid temperature, deviation of the actual rotation speed of the input member, that is, the actual input rotation speed, from an allowable rotation speed, or a situation in which a required input rotation speed cannot be achieved, can be avoided.
The second setting range may be set narrower if the temperature of the operating fluid is lower. Since the setting range of the target input rotation speed can be narrowed in accordance with the extent of decrease of the speed ratio-changing rate, it becomes possible to reliably avoid deviations of the actual input rotation speed from the allowable rotation speed, or to prevent a situation in which a required input rotation speed cannot be achieved.
Furthermore, when the temperature of the operating fluid is less than the normal temperature, a high rotation speed-side value of the target input rotation speed may be set to a value that is lower than a high rotation speed-side value set when the temperature of the operating fluid is a normal temperature. In addition, a low rotation speed-side value of the target input rotation speed, when the temperature of the operating fluid is less than the normal temperature, may be set to a value that is higher than a low rotation speed-side value set when the temperature of the operating fluid is at a normal temperature. This manner of setting the target input rotation speed makes it possible to avoid a deviation of the input rotation speed from the allowable rotation speed.
Still further, the high rotation speed-side value of the target input rotation speed calculated based on the state of running of the vehicle may be restricted via an upper limit value, or the low rotation speed-side value of the target input rotation speed may be restricted via a lower limit value. Thus, adoption of a construction in which the high rotation speed-side value of the target input rotation speed is set lower when the operating fluid temperature is lower than normal temperature than when the operating fluid temperature is normal temperature, and a construction in which the low rotation speed-side value of the target input rotation speed is set higher when the operating fluid temperature is lower than normal temperature than when the operating fluid temperature is normal temperature, makes it possible to set a suitable low rotation speed-side value or a suitable high rotation speed-side value of the target input rotation speed by suitably changing the upper limit value or the lower limit value.
Furthermore, if the continuously variable transmission is provided with a manual shift mode that allows an operator to select a transmission speed from a plurality of pre-set transmission speeds that are fixed speed ratios or have predetermined widths of speed ratios, the controller may be constructed so that if the manual shift mode is selected and the temperature of the operating fluid is less than normal temperature, the controller sets the high rotation speed-side value of the target input rotation speed alone to a value that is lower than the high rotation speed-side value set when the temperature of the operating fluid is a normal temperature. This construction makes it possible to avoid deviation of the revolution speed of a primary motor from an allowable revolution speed even when the manual shift mode is selected. Furthermore, since the range of change of the target input rotation speed is restricted only on the high rotation speed side of the target input rotation speed, an upshift request from an operator can be met.
In another embodiment, the setting range of the target input rotation speed may be changed based on the viscosity of the operating fluid instead of the temperature of the operating fluid. Modes of the invention are not limited to the above-described continuously variable transmission shift control apparatus. Other modes of the invention are, for example, a vehicle equipped with a continuously variable transmission, or a shift control method for performing a speed shift operation in the vehicle.